This invention relates to a method and apparatus for separating a vapor-gas mixture formed above a liquid in a tank, the mixture being exhausted from the tank via an exhaust line in order to relieve overpressure. In particular, the invention relates to a method and apparatus for separating a mixture of hydrocarbon vapors and gases which is to be exhausted from, e.g., an underground tank for liquid hydrocarbons, particularly motor fuels, for the purpose of relieving overpressure in the tank by exhausting material through the breather line of the tank.
In fuel filling stations, the fuel is generally stored in underground tanks which communicate with the atmosphere via at least one ventilation means, a so-called breather line. The objective is to avoid development of excessive overpressure (excessive positive gage pressure) or excessive underpressure (excessive negative gage pressure) in the tank. Particularly when fuel is being added to or removed from the storage tank, the free volume above the liquid level changes, and, in order to equalize pressure, vapor-gas mixture from the free space is exhausted to the atmosphere via the breather line, or, alternatively, air passes from the atmosphere into the tank via said breather line. The pressure in the tank can also vary due to temperature fluctuations in the tank or evaporation of liquid or fluctuations in ambient air pressure outside the tank. Aspiration of air from the surroundings to adjust underpressure in the tank does not present particular problems with regard to, e.g., environmental contamination. Exhausting of vapor-air mixture, rich in fuel vapor, to adjust overpressure in the tank, can be an environmental burden, however, and can be injurious to the health of persons in the area.
So-called active recycling systems are known, for reducing emissions of a vapor-gas mixture when filling the tank of a motor vehicle with fuel from a storage tank. These employ a pump to aspirate the vapor-gas mixture which is developed during such a filling operation, and to return said mixture to the storage tank of the filling station. As a rule, such systems are operated in a manner such that the volume of the recycled vapor-gas mixture equals the volume of the fuel withdrawn. In order to increase the effectiveness of such recycling systems, it is advantageous for the volume of the recycled vapor-gas mixture to be at least equal to the volume of the fuel withdrawn, and preferably greater than that volume. Consequently, where a greater volume of recycle is introduced to the tank than the volume of the fuel withdrawn, the pressure in the storage tank can actually be increased; this overpressure is then relieved by exhausting some of the vapor-gas mixture above the liquid into the atmosphere via the breather line. The filter element (if any) present in the breather line, under the prior art, tends to be inadequate, resulting in emission of a certain amount of fuel vapor, contained in said vapor-gas (vapor-air) mixture. Thus, while no doubt the emissions of toxic substances can be reduced in the immediate vicinity of the filling station pump apparatus (i.e. the delivery nozzle), a certain amount of fuel vapor will be emitted elsewhere at the filling station, because of the necessary pressure equalization, thereby reducing the effectiveness of the emission control system.